In recent years, we have witnessed the proliferation of wireless voice telecommunication networks. Unfortunately, due to RF channel impairments and the use of speech compression techniques, voice quality in the wireless network is not as good as it is in the wireline network. There is therefore a thrust to provide better voice quality in wireless communications.
In wireless networks, channel bandwidth is at a premium. There is therefore an advantage in compressing voice signals in digital format by an apparatus called a vocoder. The name "vocoder" stems from the fact that its applications are specific to the encoding and decoding of voice signals primarily. Vocoders are usually integrated in mobile telephones and the base stations (or a link therefrom) of the telecommunication network. They provide compression of a digitized voice signal as well as the reverse transformation. The main advantage of compressing speech is that it uses less of the limited channel bandwidth for transmission. The main disadvantage is some loss of speech quality.
One method to obtain better voice quality is to use error detection and correction techniques on the data packets transported over the network. Many methods have been proposed to detect and correct errors in the data packets being sent over the telecommunication network. A general family of control techniques is called Forward Error Correction (FEC). In FEC, redundancy is included within the data frame, which permits the detection and correction of errors. No return path is necessary when using FEC. FEC can be further divided between "Convolutional Codes" and "Block Codes". All of these techniques can be used at certain locations in the network to provide error detection and correction.
Yet, another method of error control is to add, at the transmitter site, error flags to each data packet to indicate, to the receiver, the state of the data packet. Examples of those states include indications of frames in error, of reconstructed frames, of frames replaced by signaling data, etc.